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Interesting question. "CMYK" in TIFF is a misnomer, since it's actually a way to represent any number of arbitrary inks. The 6.0 spec says that CMYK "should not be used for general image data interchange" but only for prepress applications in connection with a specific device. That suggests the answer should be no, but I don't know what sort of CMYK images, if any, are actually used in archives.

CMYK as color space of digital images was and is mainly used in the field of printing and publishing. There is no intrinsic problem with CMYK in general; it is just the way to describe colors in a subtractive color space (absorbing colors on paper), which leads to the same resulting colors as in a RGB color space. The “problem” is that CMYK does not describe color in a way a computer is working, nor our eyes. To mix colors usually all elements of an imaging workflow use a – specific, hardware dependent – RGB color space. Be it a digital camera with a CFA color filter array or be it a display that is based on three specific primaries. The reason for that: Our eyes are based on three receptors S,M and L with sensitivities in the red, green and blue range. Therefor it makes no sense to describe colors as CMYK in imaging besides in printing – and there only in the last stage were physical inks are mixed.

CMYK as color space of digital images was and is mainly used in the field of printing and publishing. There is no intrinsic problem with CMYK in general; it is just the way to describe colors in a subtractive color space (absorbing colors on paper), which leads to the same resulting colors as in a RGB color space. The “problem” is that CMYK does not describe color in a way a computer is working, nor our eyes. To mix colors usually all elements of an imaging workflow use a – specific, hardware dependent – RGB color space. Be it a digital camera with a CFA color filter array or be it a display that is based on three specific primaries. The reason for that: Our eyes are based on three receptors S,M and L with sensitivities in the red, green and blue range. Therefor it makes no sense to describe colors as CMYK in imaging besides in printing – and there only in the last stage were physical inks are mixed.

However be it RGB or be it CMYK, both spaces need a more specific definition of color, because even RGB is not a clear definition of what precise color shall be rendered. It must be defined which primaries are used to mix colors. In RGB color spaces this is defined with a standard color space as sRGB, Adobe RGB or ECI-RGB. In such standard color spaces the colors of the primaries are defined of a set of absolute XYZ values. In CMYK the situation is similar. The three colors CMY and even K (black) must be defined precisely by the definition of a color space like SWOP or FOGRA. To do so either color profiles are needed to transform color data into the proper target color space or the numbers, stored in a TIFF file must be declared – with metadata – to belong to a standard color space. If done nothing, color data is by most software regarded to be sRGB.

I think here arise some further questions: a) How problematic is it to expect a future software to be able to read and render CMYK image data properly. Must CMYK image data be converted in RGB and if yes in what target color space? sRGB for simplicity and compatibility or Lab because of the certainly not limiting size of its gamut?

At a minimum, it has to be required that Tag 332 (InkSet) have a value of 1 (CMYK) or be omitted. The only other permitted value is 2, and that informs the software that the ink set is something other than CMYK. In this case, there's no way to tell what colors the inks represent. Similarly, NumberOfInks (Tag 334) has to have a value of 4 or else be omitted.